Street is second in baseball with 14 saves and is unlikely to be traded. He does however have a bit of an injury history, and should one pop up, I'd expect Lindstrom and his 1.56 ERA to pitch the ninth. That said, there are other candidates well.
Los Angeles Dodgers
Closer: Vicente Padilla
Closer-in-waiting: Mike MacDougal
Sleeper: Scott Elbert

Padilla is dealing with forearm stiffness, so all bets are off here. Matt Guerrier got the save on Tuesday night, but it's nowhere clear that he'd be the guy should Padilla need a DL stint. You have Jonathan Broxton (elbow) and Hong-Chih Kuo (anxiety disorder) as complete unknowns, Kenley Jansen with upside but lack of control, and other assorted less-than-optimal guys. Mike MacDougal has a 1.29 ERA, so he's worth looking into despite a 4.5 BB/9. You also have a pair of rookies in Elbert and Javy Guerra who have some upside. Elbert carries his share of baggage, but all he's done since his promotion back to LA is retire all four batters he's faced, striking out three.
San Diego Padres
Closer: Heath Bell
Closer-in-waiting: Mike Adams
Sleeper: Ernesto Frieri

Bell's name should start popping up in trade rumors come June / July, and in the event he is dealt, it's a near-lock that Adams (eight holds, 19:2 K:BB in 20.2 innings). As usual, the Padres have a number of strong relievers, with six relievers having ERAs at or under 2.45 – Bell, Adams, Frieri, Luke Gregerson, Pat Neshek, and Chad Qualls.
San Francisco Giants
Closer: Brian Wilson
Closer-in-waiting: Sergio Romo
Sleeper: Ryan Vogelsong

In the event of a Marmol injury, the Cubs could choose to go with a Marshall / Wood jobshare depending on how both are pitching at the time. Alternatively, I could see a guy like Cashner getting the nod, but probably only if the injury is long-term, as it wouldn't seem practical or smart to flip Cashner from a starter's role to the bullpen and back in a compressed period of time.
Cincinnati Reds
Closer: Francisco Cordero
Closer-in-waiting: Nick Masset
Sleeper: Nick Christiani

Cordero has eight saves and a 1.86 ERA, so he's going nowhere, but he's also a guy who you look at and can see him losing the closer role at some point given he's 36 and his strikeout rate continues to drop. Masset meanwhile has allowed just one run in his last 9.1 innings and has a 9.9 K/9. Aroldis Chapman shouldn't be completely off your radar, but at this point, it's hard to see him in the closer mix any time soon. Christiani, 23, is the organization's closer in Double-A and he has a 19:4 K:BB in 19.2 innings with 80 percent of his runs allowed coming in a single appearance.
Pittsburgh Pirates
Closer: Joel Hanrahan
Closer-in-waiting: Jose Veras
Sleeper: Tim Alderson

Hanrahan's velocity is actually up year-over-year (from 96.0 mph to 97.2 mph avg fastball) yet his K/9 rate has taken a dive from 12.9 to 6.9. We won't complain too much about a 1.37 ERA, but his slider hasn't been the same pitch this year and hitters are making more contact with his fastball as a result. Behind Hanrahan, Jose Veras is next in line given his 3.06 ERA and 13.2 K/9 while longer term, the picture is less clear. Continue the rebuild and cash in on Hanrahan's trade value? Go for it and sign a high-priced closer? Or maybe turn to former top prospect Tim Alderson. Alderson has found a home in the bullpen where he has a 0.84 ERA and 22:6 K:BB in 21.1 innings for Double-A Altoona. Just a name to stash away.
Milwaukee Brewers
Closer: John Axford
Closer-in-waiting: Kameron Loe
Sleeper: Mark Rogers

Axford should continue to close for at least the next couple years, but if a fill-in is needed this year, Loe could be the guy given he leads the team with eight holds. Still that comes with a 5.12 ERA and this isn't exactly the most stable pitcher we know, so an Axford injury/demotion could mean a few saves for LaTroy Hawkins (1.04 ERA) or even Zach Braddock. Longer term, Rogers is a former top pick who just can't seem to stay healthy and put it together. After walking 22 batters in just 15 innings in Triple-A, he was demoted two levels. A move to the bullpen could revive his once-promising career, but that's pure speculation on my part.
St. Louis Cardinals
Closer: Fernando Salas
Closer-in-waiting: Jason Motte
Sleeper: Miguel Batista

I've written extensively about this situation already, so not much more to add here other than Salas is the flavor of the week, but next week's flavor could be Jason Motte or who knows who.
Houston Astros
Closer: Mark Melancon
Closer-in-waiting: Brandon Lyon (when healthy)
Sleeper: Jeff Fulchino

The Astros are 15-27, so saves are few and far between in this bullpen. Melancon has just one save since taking over for Brandon Lyon (shoulder) was sidelined. Of course he has also blown a save recently and allowed runs in his last two appearances, so his leash is probably shorter than most. David Carpenter (17:3 K:BB in 14 innings) is an interesting name in Double-A, but option No. 3 is probably Jeff Fulchino.
Atlanta Braves
Closer: Craig Kimbrel
Closer-in-waiting: Jonny Venters
Sleeper: Cory Gearrin

The Marlins are 24-16 and Nunez leads the majors with 15 saves, so unless something goes south prior to the trade deadline, it appears Nunez is staying in Florida, at least for this year. Should things change, there really isn't an obvious successor, as last year's surprise veteran, Clay Hensley, has struggled some (2.89 ERA but a 7:8 K:BB). Ryan Webb has longer term potential, but his 11:8 K:BB in 18.2 innings is less than inspiring. Hensley probably would get the short-term nod, but longer term, I like Webb and Triple-A closer Jose Ceda (0.50 ERA and 21:6 K:BB in 18 innings).
New York Mets
Closer: Francisco Rodriguez
Closer-in-waiting: Jason Isringhausen
Sleeper: Taylor Buchholz

The Mets will do what they can to ensure that Rodriguez's $17.5 million option (based on games finished) doesn't vest. David Wright's back injury helps, as does the fact this is a sub-.500 team, but he still has 12 saves despite all that. Next up should Rodriguez get hurt or traded would probably be Isringhausen, though the Mets could also recognize that he's also not part of their future and turn to Buchholz. Buchholz has a 1.66 ERA and 24:7 K:BB in 21.1 innings as he's recovered nicely from 2009 Tommy John surgery. They may not give him the 2012 closer job, but he's probably the leading in-house candidate currently capable of filling the slot.
Philadelphia Phillies
Closer: Ryan Madson
Closer-in-waiting: Jose Contreras
Sleeper: None

Madson has six saves and a 0.56 ERA, so he's likely the guy to hold the job until Brad Lidge returns in late-June (or later) and proves his shoulder is sound. That said, Charlie Manuel bypassed Madson for Jose Contreras at the beginning of the year, so once Contreras returns from an elbow injury, he's a threat to Madson's ability to pile up saves. Contreras probably returns in a setup role, but if he proves he's 100 percent, he could be saving games by the end of May.
Washington Nationals
Closer: Drew Storen
Closer-in-waiting: Tyler Clippard
Sleeper: Cole Kimball

Storen is locked and loaded here, something that should continue into the next decade. Clippard is the would-be fill-in while Kimball is the sleeper pick here. He's a guy with a 12.3 K/9 a year ago in Double-A despite occasional lapses in his control.
American League

Walden has blown two of his last three save opportunities and has allowed runs in four of six appearances. That said, he's still missing bats and he has a 9.8 K/9 on the year, so for now Walden is in no danger of losing his closer job. Rodney has a 14:13 K:BB, so while he has the experience, it could be Scott Downs or even Kevin Jepsen that gets the call in the event Walden goes down. Jepsen had an ugly outing Wednesday (1 IP, 5 ER), so he probably isn't in the mix in the short term.
Oakland A's
Closer: Brian Fuentes
Closer-in-waiting: Andrew Bailey
Sleeper: Grant Balfour

Balfour leads the team with nine holds, so he could be the guy tapped for saves should Bailey (forearm) suffer another injury down the road. Bailey is expected to return in about 10 days and we should see him slide into the closer role shortly thereafter.
Seattle Mariners
Closer: Brandon League
Closer-in-waiting: Jamey Wright
Sleeper: Aaron Laffey

David Aardsma is looking less likely to be factor this year with hip and elbow injuries, leaving the closer role in League's lap. League though has a string of ugly outings recently that have resulted in a 6.88 ERA. Failed starters Wright and Laffey appear to be next in line, with the right-handed Wright and his 1.88 ERA garnering the most FAAB attention. David Pauley is your deep, deep sleeper here given his 1.16 ERA.
Texas Rangers
Closer: Neftali Feliz
Closer-in-waiting: Andrew Oliver
Sleeper: Tanner Scheppers

Scheppers is currently recovering from a back injury, but he's a former first-round pick with very good upside, so assuming the Rangers slot him as a RP long-term, he's worth monitoring. On the current roster, a Feliz injury likely results in Darren Oliver and perhaps Mark Lowe sharing saves.
Chicago White Sox
Closer: Sergio Santos
Closer-in-waiting: Jesse Crain
Sleeper: Dan Remenowsky

Santos has yet to allow a run this season in 20 innings, so he's showing no signs of wanting to relinquish his job. Beyond Santos, the White Sox have struggling lefties Matt Thornton and Chris Sale and probable closer-in-waiting Jesse Crain. Crain has a 2.66 ERA and more K's than IP, so he's probably be worthy closer if needed. A deep sleeper could be Remenowsky, who has a 27:1 K:BB in 20.1 innings for Double-A Birmingham this year and a 169:29 K:BB in 102.2 innings from 2009-2010. Video game numbers.
Cleveland Indians
Closer: Chris Perez
Closer-in-waiting: Tony Sipp
Sleeper: Nick Hagadone

Perez may not even be the best reliever on his own team with that last name, but he has 10 saves and plenty of security. Sipp has a 1.37 ERA, but his 14:7 K:BB in 19.2 innings is fine, but not exactly eye-popping. Longer term, Hagadone has an interesting pedigree as a former top prospect with the Red Sox, and he appears to have found a home in the bullpen with a 1.66 ERA in 11 games for Double-A Akron.
Detroit Tigers
Closer: Jose Valverde
Closer-in-waiting: Ryan Perry
Sleeper: Al Albuquerque

With Joaquin Benoit out of the eighth inning mix due to a 7.98 ERA, the field is wide open behind Valverde. Albuquerque has been a revelation with his 15.5 K/9 while former #1 pick Perry is still a potential closer of the future despite a 7.27 ERA.
Kansas City Royals
Closer: Joakim Soria
Closer-in-waiting: Aaron Crow
Sleeper: Jeremy Jeffress

Lots of talent in the Royals bullpen and roster in general, but Soria is clearly going to be the guy barring a trade or injury. Crow seems to have settled in the bullpen long-term, so he'll battle Jeffress for that "closer of the future" label. Short-term however, it's likely to be Crow given his 0.89 ERA.
Minnesota Twins
Closer: Matt Capps
Closer-in-waiting: Joe Nathan
Sleeper:

Capps and his 0.78 WHIP continues to be the preferred option in Minnesota while Nathan has supposedly moved back into a late-inning role despite a 7.43 ERA. Nathan could eventually reclaim his old job, but it's clear he's going to have to prove himself. Longer term, Cole DeVries is 26 and pitching in Double-A, but it's hard to ignore that he's closing games there with a 31:5 K:BB in 24.2 innings.
Boston Red Sox
Closer: Jonathan Papelbon
Closer-in-waiting: Dan Bard
Sleeper: Michael Bowden

It's hard to imagine a scenario in which the Red Sox trade Papelbon this year, but it's entirely possible he signs a big-money deal with another team this offseason. Bard is clearly next in line and my pick for the team's 2012 closer. An interesting fall-back could be Bowden, who currently has a 1.59 ERA and 28:4 K: BB in 22.2 innings for Triple-A Pawtucket.
Baltimore Orioles
Closer: Kevin Gregg
Closer-in-waiting: Koji Uehara
Sleeper: Jose Diaz

Gregg's K: BB is an uninspiring 14:12 in 16.1 innings, so he remains a candidate to lose his job at some point this year. In the meantime, Uehara has a 2.50 ERA and solid 22:5 K: BB in 18 innings, so he could easily be in the mix sometime soon. Longer term, Jose Diaz has a 1.62 ERA and 12 saves for Double-A Bowie and yes; this is the guy a.k.a. "Jumbo."
New York Yankees
Closer: Mariano Rivera
Closer-in-waiting: David Robertson
Sleeper: Philip Hughes

We know all about Rivera of course, but should he get hurt or turn back into his human / non-alien form, the elbow injury to Rafael Soriano leaves the Yankees' closer situation up in the air. Joba Chamberlain and Robertson would vie for closing duties, and with Robertson sporting a 1.62 ERA and 14.0 K/9, he's got the slight edge. Longer term, the Yankees are likely to spend big on a closer once Rivera retires, but perhaps they consider moving Hughes into that role at some point. Bash away.
Tampa Bay Rays
Closer: Kyle Farnsworth
Closer-in-waiting: Joel Peralta
Sleeper: Cory Wade

Farnsworth is looking like he'll keep the job for the balance of this season; with Peralta likely the guy the team would turn to should Farnsworth get hurt. Long term, Cory Wade was once a very good reliever for the Dodgers, but injuries derailed his career. Healthy now, Wade has a 0.74 ERA and 24:4 K: BB in 24.1 innings for Triple-A Durham.
Toronto Blue Jays
Closer: Frank Francisco
Closer-in-waiting: Jon Rauch
Sleeper: Carlos Villanueva

Francisco won't top any closer cheat sheets right now, but he's the preferred option over Rauch. The Jays also have three other guys with sub-2.00 ERAs in Villanueva, Jason Frasor, and Casey Janssen, so Plan B's are abundant. Thinking into the future, Ale x Farina (1.26 ERA in Double-A, former 3rd round pick) could be an option in 2012 and beyond.
Regan, a four-time Fantasy Sports Writers Association award winner, was named the 2010 Fantasy Baseball Writer of the Year.